Various resins have been used in the manufacture of laminated wood and paper structures such as laminated veneer lumber (LVL), including plywood, chipboard, particleboard, and paper tubing. An important consideration in the choice of an adhesive for manufacturing said articles is the curing rate and the temperature required for promoting curing. In many cases the billets to be bonded are over three centimeters in thickness. In such cases, the penetration of heat is very slow and requires the consumption of substantial time and energy.
In practice, the most widely used adhesives are resorcinol/phenol formaldehyde type resins which provide excellent adhesion but nevertheless require heat activation and considerable amounts of time to cure. Another drawback encountered in their use is their black color which contributes to an aesthetically less attractive end product. A further drawback to the use of phenolic resin adhesives is that they cannot be used effectively with substrates having high moisture contents. Since many wood substrates have moisture contents of 10% by weight or above, it is necessary to dry the substrates so that they have about a 5% by weight moisture content. When wood substrates are treated to reduce their moisture content, it results in a loss of wood volume and increases the cost and time of producing wood products which employ the phenolic resins.
Isocyanate crosslinked latexes have also been employed as adhesives but their cure rate is relatively slow even with the application of heat. Polyisocyanates have been employed as adhesives for the manufacture of cellulosic materials in the formation of waferboard, fiberboard and paper laminates. Such applications employing polyisocyanates normally require temperatures up to 200.degree. C. or more for curing in order to develop acceptable physical bonding strengths.
The use of polyisocyanates and imino- and enamino-functional compounds in reaction injection molding systems is known in the art. For example, U.S. Pat. No. 4,794,129 discloses a reaction system for making a polymer by a reaction injection molding process wherein the system is comprised of an organic aromatic polyisocyanate and an isocyanate reactive composition comprising at least one imino-functional compound. U.S. Pat. No. 4,935,460 discloses a reaction injection molding system in which the A-component is an organic polyisocyanate and the B-component is an isocyanate reactive component comprising the reaction product of an isocyanate-terminated polyurethane prepolymer and an imino-functional or enamine-containing compound. Neither patent, however, discloses that its reaction systems can be employed as adhesives or sealants.
Also, U.S. Pat. No. 3,865,791 discloses the preparation of a prepolymer from a mixture comprising, in part, a monoenamine and a polyisocyanate. This prepolymer may then be combined with further polyisocyanate and moisture-cured. U.S. Pat. No. 4,853,454 discloses a single component moisture-curable polyurethane system comprising a polyurethane prepolymer and a polyaldimine. The compositions of these patents may be used as coatings, castings, patchings, and cements. However, neither of these patents disclose the use of the compositions with lignocellulosic or cellulosic materials in the preparation of composite products, such as particleboard.
There is a need for a cold curable resin system which fully cures in a short period of time at room temperature yet has an adequate pot life suitable for commercial production. Ideally, such an adhesive system could be cured within 30 minutes at room temperature, have a working pot life of greater than 5 minutes and provide a bonding strength equal to or greater than the prior art resin systems.
There is also a need for an adhesive which can be employed with high moisture containing substrates, particularly lignocellulosic or cellulosic substrates.
It is, therefore, an object of this invention to provide an adhesive/sealant reaction system which can be cold cured.
It is another object to provide an adhesive/sealant reaction system which can be cured at or near room temperature within a relatively short period of time.
It is a further object to provide an adhesive/sealant reaction system which has a working pot life of greater than 5 minutes.
It is also an object to provide an adhesive system which provides a bonding strength equal to or greater than the bonding strength of prior art resin systems.
It is yet another object to provide an adhesive system which may be employed with relatively high moisture containing substrates, such as lignocellulosic or cellulosic substrates.
These and other objects are achieved by the reaction systems of the present invention.